This application is based on and claims the benefit of German Patent Application No. 198 31 719.0 filed Jul. 15, 1998, which is incorporated by reference herein.
The invention is based on a process for producing planar waveguide structures as well as a waveguide structure according to the generic class of the independent claims.
The literature, for example, xe2x80x9cOptische Telekommunikationssysteme,xe2x80x9d [Optical Telecommunications Systems] Publisher Hagen Pultsch, describes manufacturing processes (see pp. 117 ff.) and waveguide structures produced by these manufacturing processes (see pp. 221 ff.).
To guide light in an optical waveguide the refractive index of the waveguide must be greater than the refractive index of its environment. In principle, the planar waveguide comprises a dielectric circuit carrier, a substrate with a first refractive index n1, on which is located a waveguiding dielectric with a refractive index n2. The cover layer is either air or an additional dielectric with a refractive index n3. With this structure, light can be guided along the boundary surfaces of the waveguide due to repeated total reflections. Examining the field strength distribution of a light wave in a strip line, one can see that the field along the field boundary to the neighboring medium does not abruptly die down to zero. The waveguides behave as open waveguides. If the refractive index differences between the waveguides of the structure and the environment are sufficiently large, the field spurs extend only slightly out of the waveguiding structure. To obtain optimum waveguidance it is desirable to keep the refractive index difference large and, in particular, to make the structure along the boundary clean during the manufacturing process.
Various problems are encountered when manufacturing waveguide structures in glass or silicon material by means of known processes such as glass deposition from the vapor phase. Under the action of high temperatures, the waveguides are frequently deformed and are subject to stresses. Both problems result in poor light guidance within the waveguide. Applying an upper cover layer frequently causes bubble structures, which also impair the quality of the waveguide. Furthermore, out-diffusion of doping materials from the optical core into the surrounding layers occurs. As a result, the jump in the refractive index along the boundary layer is not sharp enough to bring about clean waveguidance.
The process according to the invention with the characteristic features of the independent claim has the advantage that it describes a simple way to structure the optical core such that an optimal symmetrical sphere surrounds the core and prevents out-diffusion of doping materials. Furthermore, the symmetrical sphere consisting of a fluoride-containing layer has a stabilizing effect on the shape of the core and thus reduces stress in the waveguide structure.
The measures set forth in the subclaims provide an advantageous further development and improvement of the process defined in the independent claim.
The process is particularly advantageous for different waveguide structures based on glass or silicon, whereby the layers are created, for example, by flame hydrolysis. The core structure can advantageously be structured by a reactive ion etching process.
Furthermore, it is advantageous to produce the fluoride-containing layer by applying a fluoride-containing liquid. This has moreover the advantage of creating a very thin fluoride layer, which has a markedly changed refractive index compared to the material of the waveguide. A layer thickness on the order of 150 nm is optimal. Applying a fluoride-containing liquid has the additional effect of cleaning the structure material.
The inventive waveguide with the characteristics of the independent claim has the advantage that it permits very clean wave guidance, whereby the manufacturing process itself is not burdened by costs and complex work operations.